The present invention relates to a process for preparing methylenedianiline by reacting aniline with formaldehyde in the presence of acid catalysts, the mixtures which can be prepared by this process comprising methylenedianiline, a process for preparing polyisocyanates by phosgenation of these mixtures comprising methylenedianiline, and polyisocyanates obtainable in this manner.
The preparation of methylenedianiline (also termed MDA below), is generally known and is customarily carried out by continuous or batchwise reaction of aniline with formaldehyde in the presence of acid catalysts. In this reaction, whose main product is 4,4xe2x80x2-MDA, the unwanted byproduct N-methyl-MDA is formed to a small extent. This byproduct is disadvantageous, in particular in the subsequent reaction of the MDA with phosgene to prepare methylenebis(phenyl isocyanate), also termed MDI, since the N-methyl-MDA is the precursor compound for chlorinated byproducts in the MDI and chlorine contents in the MDI as low as possible are sought.
To decrease N-methyl-MDA as byproduct in the preparation of MDA, various processes are known.
Thus, U.S. Pat. No. 5,286,760, for continuous MDA preparation, describes partial neutralization of the reaction mixture between the stage of condensation of two molecules of aniline and one molecule of formaldehyde and the subsequent rearrangement of the intermediate aminobenzylamines, abbreviated as ABA, to give MDA.
EP-A 451 442 and DD-A 238 042 disclose, for a continuous process, the addition of formaldehyde over a plurality of process stages.
Processes for decreasing the byproduct are also known for batchwise processes. DD-A 295 628 describes the addition of formaldehyde in two steps during the condensation stage, in the first addition the main amount of formaldehyde being added at a low temperature and the second addition of the remaining formaldehyde being performed at the same or higher temperature.
A disadvantage in these processes is the insufficient decrease of the N-methyl-MDA content in the product mixture, so that there is still a need for improvement.
Processes for preparing MDI from MDA by phosgenation are generally known.
It is an object of the present invention to develop a process for preparing methylenedianiline by reacting aniline with formaldehyde in the presence of acid catalysts which minimizes the N-methyl-MDA content as an unwanted byproduct. Such an MDA should be used, in particular, in an improved process for preparing methylenebis(phenyl isocyanate) (MDI), which should make accessible an MDI having improved properties, in particular a low chlorine content and/or a light color, in particular in the crude MDI which, in addition to the monomeric MDI, also comprises polymeric MDI, and/or should be made accessible in the polymeric MDI.
We have found that this object is achieved according to the invention, in a semicontinuous process, by introducing aniline with or without acid catalyst, feeding formaldehyde with or without acid catalyst through a mixing element into a circuit in which aniline with or without acid catalyst and with or without previously added formaldehyde is circulated and, after feeding in at least 50% of the total amount of formaldehyde to be fed in, heating the reaction mixture to a temperature above 75xc2x0 C.
This novel procedure permits a higher content of higher MDA oligomers to be obtained than is possible by a continuous procedure at high molar ratios of aniline to formaldehyde without recycling the MDA. By the process according to the invention, minimizing the content of unwanted byproducts is possible.